1. Field of the Invention
This invention relates to a method of sensing a vehicle which uses a sensing loop coil. Also, this invention relates to an apparatus for sensing a vehicle which uses a sensing loop coil.
2. Description of the Related Art
Some vehicle sensing apparatuses include a loop coil provided on a road or buried in the ground under a road. The loop coil forms a part of a resonance circuit in an oscillation circuit. Thus, the inductance of the loop coil determines the frequency of the output signal of the oscillation circuit. When a vehicle passes through a given region or a sensing area near the loop coil, the inductance of the loop coil varies and the frequency of the output signal of the oscillation circuit also varies. Such a variation in the output signal of the oscillation circuit is detected as an indication that a vehicle passes through the sensing area (the given region).
A prior-art vehicle sensing apparatus having a loop coil detects the deviation .DELTA.f of the frequency of the output signal of an oscillation circuit from a reference frequency. When the frequency deviation .DELTA.f exceeds a first threshold value, it is decided that a vehicle passes through a sensing area. The inductance of the loop coil, that is, the frequency of the output signal of the oscillation circuit, tends to vary in response to an environmental change such as a change in atmospheric temperature or a start of rain. In view of this fact, the prior-art vehicle sensing apparatus implements adaptive control of the reference frequency. Specifically, there is set a second threshold value smaller than the first threshold value. An allowable range for the frequency deviation .DELTA.f is provided in connection with the second threshold value. In the case where the frequency deviation .DELTA.f moves out of the allowable range, a decision is made as to whether or not the frequency deviation .DELTA.f exceeds the second threshold value during a given time interval t0 from its movement out of the allowable range. If it is decided that the frequency deviation .DELTA.f does not exceed the second threshold value until the end of the given time interval t0, the reference frequency is corrected. The given time interval t0 is referred to as a follow-up time for an environmental change. Generally, the follow-up time is set to 1 second.
The prior-art vehicle sensing apparatus has a problem as follows. Vehicles in a traffic jam tend to slowly pass through the sensing area. In some of these cases, the frequency deviation .DELTA.f remains lower than the second threshold value for longer than the follow-up time so that the reference frequency is corrected. This correction of the reference frequency is caused by the traffic jam rather than an environmental change to be considered.